1. Field of the Invention
The present invention relates to an antiskid control apparatus for controlling the slip of a wheel which occurs upon application of the brake in a vehicle. To be more specific, the present invention relates to an antiskid control apparatus for increasing brake pressure by using a hydraulic pressure generated by a master cylinder.
2. Description of the Related Art
A conventional antiskid control apparatus comprises a slip-state detecting unit for detecting slip states of wheels during braking, a master cylinder for generating a hydraulic pressure according to the application of the brake pedal and a brake-pressure adjusting unit for increasing and decreasing brake pressure applied to the wheels so that slip states detected by the slip-state detecting unit are controlled to predetermined states. The brake-pressure-adjusting unit increases the brake pressure by using the hydraulic pressure generated by the master cylinder. Due to this structure, the antiskid control unit can prevent skidding and satisfactorily shorten a braking distance.
However, it is difficult to detect slip states of wheels at low speeds because the accuracy of signal detection by wheel speed sensors deteriorates. In order to solve this problem, antiskid control ceases when the speed of the vehicle drops below a predetermined value. When the antiskid control ends, brake fluid flowing from the master cylinder is supplied to a brake unit (wheel cylinders) in the conventional manner.
In the antiskid control unit described above, however, movement of the brake pedal during antiskid control is restricted because the brake-pressure adjusting unit increases the brake pressure by using the hydraulic pressure generated by the master cylinder and decreases it while cutting off the conduit connected to the master cylinder. Then, when antiskid control ceases, the brake fluid flowing from the master cylinder is supplied to the brake unit without antiskid control. As a result, a driver feels an abrupt difference in the sensitivity of the brake pedal and in the force required to operate the brake pedal. To the driver, it appears that the necessary force to apply to the brake pedal decreases abruptly as soon as antiskid control stops. Therefore, at low speeds, the feedback to the driver from application of the brake is unacceptable because it is not consistent through non-antiskid control and antiskid control.
It is thus a first object of the present invention to provide an antiskid control apparatus that generates a consistent and acceptable feedback feeling to a driver during a brake-pedal operation, at low speeds and which also prevents the driver from receiving inconsistent feedback throughout the braking process.
In addition, typically the restriction on the movement of the brake pedal during antiskid control causes the brake pedal to move a predetermined length only after a certain amount of time. This causes awkward feedback to the driver who feels that he or she cannot depress the brake pedal because the pedal hardly moves. To contrary, the brake pedal may be allowed to move the predetermined length in a much shorter amount of time. This causes the driver to receive feedback from the brake pedal indicating that applying force to the brake pedal will push the brake pedal to the bottom of its stroke. It is thus a second object of the present invention to provide an antiskid control apparatus that provides acceptable and consistent brake-pedal operation feedback to the driver by adjusting the time spent for the movement of the brake pedal.